The present invention is directed to pulp processing, and more particularly, to apparatus and method for the displacement washing and pressing of pulp.
In the processing of wood chips to pulp, the chips are placed in a digester, which is a large vessel containing a solution referred to as "cooking liquor". The pulp and cooking liquor are then heated under pressure in order to remove the resins and to dissolve the lignin which holds together the desired cellulose fibers in the wood. The pulp, as it is discharged from the digester, contains a high proportion by weight of the cooking liquor. The liquor must be removed from the pulp before the pulp can be used in the manufacture of paper.
A conventional technique for removing liquor from pulp is to pass the pulp slurry through a screw press extractor, wherein the liquor is removed, or "expressed", from the slurry and, to a significant extent, the pulp fibers. Thereafter, the compressed pulp is conveyed to another piece of equipment, wherein it is mixed with water or other chemically active liquid, which loosens residual liquor or other undesirable components of the compressed pulp, so that they may be washed away. The washed pulp is then conveyed to a second screw press or the like, where the wash liquid is removed and the repressed pulp conveyed for further processing.
U.S. Pat. No. 3,256,808, "Screw Press Extractor" illustrates some common features of this type of screw press extractor, including a pressure screw which rotates within a cylindrical housing, the slurry thus being forced to pass through a narrowing restriction between the screw and the housing, such that the extracted liquid is removed from the housing through a housing screen.
U.S. Pat. Nos. 4,088,528 and 4,214,947, illustrate a pressure screw grinder and digester, respectively, wherein grinding and/or delignification is achieved by passing the raw material between interpenetrating helicoidal surfaces driven synchronously in rotation inside a casing. A braking zone is provided in which the pitch of the screw surfaces is reversed, thereby effectively forming a restriction. In these patents, the differences in pitch in part define different zones, where different chemical and/or mechanical processes are carried out. In one embodiment, the cellulosic material in the form of chips, it is passed in succession through a first braking zone which causes a first compression of the material, a subsequent zone where the material is brought into contact with a reagent, and another braking zone which causes a second compression. The first compression stage causes the expulsion, along the screw axis, of water present in the material and each subsequent compression causes the expulsion of any spent reagent and of residual liquors in the material.
In U.S. Pat. Nos. 3,533,510 and 3,911,809, a screw press dewatering apparatus is shown wherein a number of flaps are provided at the downstream end of the device immediately upstream of the discharge port, such that the back pressure can be adjusted at different annular positions around the last restrictor, thus providing a more uniform output of dewatered material.
U.S. Pat. No. 3,067,672 discloses a screw press apparatus and method wherein a washing and mixing station is positioned upstream of the compression and extraction section, these sections having different screws, oriented on different axes.